Sodium intake alters the effects of norepinephrine on the renin system and the kidney

Studies of salt and stress sensitivity on arterial pressure in renin-b deficient mice

Excessive sodium intake is known to increase the risk for hypertension, heart disease, and stroke. Individuals who are more susceptible to the effects of high salt are at higher risk for cardiovascular diseases even independent of their blood pressure status. Local activation of the renin-angiotensin system (RAS) in the brain, among other mechanisms, has been hypothesized to play a key role in contributing to salt balance. We have previously shown that deletion of the alternative renin isoform termed renin-b disinhibits the classical renin-a encoding preprorenin in the brain resulting in elevated brain RAS activity. Thus, we hypothesized that renin-b deficiency results in higher susceptibility to salt-induced elevation in blood pressure. Telemetry implanted Ren-b^Null and wildtype littermate mice were first offered a low salt diet for a week and subsequently a high salt diet for another week. A high salt diet induced a mild blood pressure elevation in both Ren-b^Null and wildtype mice, but mice lacking renin-b did not exhibit an exaggerated pressor response. When renin-b deficient mice were exposed to a high salt diet for a longer duration (4 weeks), there was a trend for increased myocardial enlargement in Ren-b^Null mice when compared with control mice, but this did not reach statistical significance. Multiple studies have also demonstrated the association of environmental stress with hypertension. Activation of the RAS in the rostral ventrolateral medulla and the hypothalamus is required for stress-induced hypertension. Thus, we next questioned whether the lack of renin-b would result in exacerbated response to an acute restraint-stress. Wildtype and Ren-b^Null mice equally exhibited elevated blood pressure in response to restraint-stress, which was similar in mice fed either a low or high salt diet. These studies suggest that mechanisms unrelated to salt and acute stress alter the cardiovascular phenotype in mice lacking renin-b.

Alpha 1-adrenergic blockade and cardiovascular pressor responses in essential hypertension

The effects of selective alpha 1-adrenergic blockade with terazosin on blood pressure and cardiovascular pressor responsiveness were assessed in 17 subjects with mild to moderate essential hypertension (mean age, 48 +/- 2 [SEM] years). As compared with a 2-week placebo period, 8 weeks of terazosin treatment (mean dose, 10.5 +/- 1.7 mg/day) caused a fall of supine (from 153/103 +/- 3/2 to 143/96 +/- 4/2 mm Hg; p less than 0.025) and upright (from 145/106 +/- 4/2 to 131/94 +/- 5/3 mm Hg; p less than 0.01) arterial pressure; a marked blunting of cardiovascular pressor responsiveness to norepinephrine, as judged from the pressor dose (from 73 +/- 9 to 2156 +/- 496 ng/kg/min; p less than 0.02) and from the rightward shift (p less than 0.01) of the plasma concentration-blood pressure response curve; and a slight increase in plasma norepinephrine concentration (from 37.7 +/- 3.3 to 52.2 +/- 7.8 ng/dl; p less than 0.01). Heart rate, body weight, exchangeable sodium, blood volume, and norepinephrine plasma clearance; plasma epinephrine, renin, angiotensin II, and aldosterone levels; the relationships between angiotensin II-induced increases in arterial pressure or plasma aldosterone and the concomitant increments of plasma angiotensin II; and heart rate responsiveness to isoproterenol did not change significantly after terazosin treatment. These findings suggest that the fall of arterial pressure induced by selective alpha 1-adrenergic blockade in subjects with essential hypertension is associated with, and probably explained by, inhibition of alpha 1-mediated, noradrenergic-dependent vasoconstriction. alpha 1-Adrenergic receptor antagonism did not modify body sodium concentration, the adrenomedullary component of the sympathetic nervous system, angiotensin II levels, or beta-adrenergic dependent mechanisms.

The effects of rapid saline infusion on sodium excretion, renal function, and blood pressure at different sodium intakes in man

To examine the effects of increasing dietary sodium intake on natriuresis, filtration rate, and renal blood flow following rapid volume expansion, we infused 2-liter normal saline over 2 hr into normal men in balance at 10, 300, 600, and 800 mEq/day sodium intake. Natriuresis and kaliuresis were related to prior sodium intake. Fractional excretion of sodium (6%-7%) was maximal at the 600 mEq/day sodium intake and increased no further at the 800 mEq/day sodium intake. Although blood pressure increased with rapid saline infusion, natriuresis and blood pressure were not associated. Creatinine clearance decreased or remained constant, while PAH clearance decreased during saline infusion at each level. The data suggest that although natriuresis following rapid saline infusion is dependent upon prior sodium intake, under given circumstances it may be independent of glomerular filtration rate, renal blood flow, or blood pressure.